With appropriate electrical loading, photovoltaic solid state semiconductor devices, commonly known as solar cells, convert sunlight into electrical power by generating both a current and a voltage upon illumination. The current source in a solar cell is the charge carriers that are created by the absorption of photons.
CIGS-Based Solar Cells
Copper ternary chalcogenide compounds and alloys are efficient light-absorbing materials for solar cell applications. Their efficiency is due to their direct and tunable energy band gaps, very high optical absorption coefficients in the visible to near-infrared (IR) spectrum range and high tolerance to defects and impurities. Copper indium-gallium-selenium/sulfur (CIGS) thin film solar cells provide the advantages of low-cost, high-efficiency, long-term stability, superior performance under weak illumination, and desirable resistance to radiation. For useful background material, refer, for example, to H. W. Schock et al., “CIGS-based Solar Cells for the Next Millennium,” Prog. Photovolt. Res. Appl. 8, 151-160 (2000).
Unlike the basic silicon solar cell, which can be described as a simple p-n junction device, CIGS based solar cells comprise a more complex heterojunction system. Solar cells based on CIGS have achieved the highest efficiency of existing thin film solar cells.
A cross-sectional view of an exemplary CIGS device in accordance with a prior art embodiment is shown in FIG. 1. The various layers of the solar cell are deposited on a substrate 110. Solar cells based on p-type CIGS absorbers are typically fabricated on glass, polymer, stainless steel or other substrates 110 using various deposition techniques known in the art. Incident sunlight 180 is partially blocked by the metallic grid shown as contact elements 170, which covers approximately 5% of the surface of the device. These contact elements 170 can comprise Ni/Al fingers or other appropriate contact elements. The incident sunlight 180 is partially reflected by the surface of the transparent conducting-oxide (TCO) layer 150 and 160, shown as a i-ZnO/ZnO:Al layer, due to the difference in the index of refraction. Some short-wavelength photons are absorbed in the n-CdS layer 140. Most of the sunlight, however, enters the semiconductor and is absorbed in the CIGS absorber layer 130. The CIGS absorber layer 130 is shown disposed on molybdenum (Mo) layer 120, which is disposed on a stainless steel substrate 110 as shown. Other substrates known in the art are expressly contemplated, including glass and polymers.
The front metal contact fingers (Ni/Al) 170 are optional and are not required for operation of the photovoltaic device. The ZnO layers 150&160 and CdS layer 140 typically comprise n-type material, and the CIGS 130 layer typically comprises p-type material. The semiconducting junction is formed at or proximate to the CdS-CIGS (n-p) interface. Electrons that are generated within the junction-field region or within about one diffusion length of the n-p junction will generally be collected.
According to standard prior art CIGS thin film solar cells, a highest efficiency of 19.9% has been achieved with an effective area of 0.42 cm2 prepared by the so-called three-stage co-evaporation process. For useful background material on efficiencies of CIGS solar cells, refer, for example, to I. Repins et. al., “19.9% efficient ZnO/CdS/CuInGaSe2 solar cell with 81.2% fill factor”, Prog. Photovolt: Res. Appl. 2008; 16:235-239.
QD-Enhanced Solar Cells
The efficiency of a solar cell can also be enhanced when a quantum dot (QD) effect is applied to a solar cell, thereby significantly improving the energy conversion rate thereof. Quantum dots have bandgaps that are tunable across a wide range of energy levels by changing the size (i.e. diameter) of the quantum dots. The quantum dot effect achieved by a quantum dot solar cell generally relates to an impact ionization effect and an Auger recombination (AR) effect.
The impact ionization effect occurs in semiconductor material, when energy of two bandgaps is provided from external, excited electrons can exist in form of hot electrons. When the hot electrons are transited form high energy level to low energy level excitation state, the released energy can excite another electron from a valence band to a conduction band, and such phenomenon is referred to as the impact ionization effect. According to the impact ionization effect, one high energy photon can excite two or more hot electrons.
The other effect is an Auger recombination (AR) effect relative to the impact ionization effect. The AR effect refers to the energy released in the semiconductor material due to recombination of hot electrons and holes can excite another hot electron to transit to a higher energy level, thereby prolonging a lifetime of the hot electron in the conduction band.
When the semiconductor material displays a quantum dot size, the continuous conduction band is gradually split into small energy levels, so that the cooling speed of the electrons is slowed down, and therefore the impact ionization effect and the Auger recombination effect can be effectively utilized. According to theoretical calculations, the tradition single junction solar cell only can achieve 31% energy conversion efficiency, and if combining with the impact ionization and Auger recombination effects, the maximum theoretical efficiency of the solar cell can be 66%, which confirms the potential ability to use the quantum dots in the solar cell.
Solar cells made from photosensitive nanoparticles show very low efficiencies. Nanoparticles are very efficient in generating electron-hole charge pairs when exposed to sunlight. The primary reason for the low efficiencies is charge recombination. To achieve high efficiencies in a solar cell the charges are desirably separated as soon as possible after they are generated. Charges that recombine do not produce any photocurrent and hence do not contribute towards solar cell efficiencies. Charge recombination in nanoparticles is primarily due to two factors: (a) Surface states on nanoparticles that facilitate charges recombination and (b) Slow charge transport. In nanoparticles/QD solar cells, the charge recombination is faster as compared to charge transport.
Electricity produced by a solar cell is expensive due to high solar cell module cost. In order to significantly reduce the cost of solar electricity, it is desirable to both increase cell efficiency and to significantly reduce the costs of photovoltaic (PV) module fabrication. A thin film form of cell reduces the fabrication cost, but yields relatively lower efficiency compared to a single crystalline wafer based cell. Thus, it is desirable to provide a system and method to enhance efficiency of the solar cell.